Index fund.

In plain English

An index fund is built to match a market index (like the S&P 500) by holding all the same companies the index holds, in the same proportions. There's no manager picking stocks. The fund just owns what the index owns, and changes when the index changes. Because there's no expensive research team, fees are tiny.

01Why it matters

Decades of data show that the vast majority of professional stock pickers underperform a basic index fund over the long run, after fees. The simplest, lowest-fee approach is also usually the best one for the average person.

02The math, step by step

An S&P 500 index fund owns shares in roughly the 500 largest US companies, weighted by size. If you put $1,000 in, the fund spreads that across all 500. You're not betting on a single winner. You're betting that, on average, the largest US companies will keep growing over decades. That bet has paid off historically, though never in a straight line.

Illustrative example. The amounts here are hypothetical, chosen to show how the math works, not real quoted rates or figures.

03What this is NOT